This application claims the priority of German Patent Document No. 102 18 669.3-45, filed Apr. 26, 2002, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a fuel cell system having a fuel cell unit, and to a method for voltage monitoring for a fuel cell system having a fuel cell unit.
Germany Patent DE 198 27 880 C1 discloses a circuit device for a fuel cell unit, by means of which the individual fuel cells can be bridged in the form of a low-impedance parallel circuit. Furthermore, a method for operating such a circuit arrangement is disclosed, with the fuel cells being bridged when the magnitude of their voltage falls below a specific threshold value, or when the mathematical sign of their voltage has changed from that during normal operation, or else when the overall fuel cell system is intended to be discharged.
A method for monitoring one or more cells in a fuel cell unit is known from the generic German Patent DE 100 55 291 A1. The monitored voltage is compared with a first and a second preselected voltage value as a function of the load, an electrical component, with the second preselected value being less than the first preselected value. If one of the preselected values is undershot or if a low-voltage event is detected, then an adjusting action is initiated. When an adjusting action is initiated, an external load is reduced, or the supply of power to the external load is ended.
During normal operation, a fuel cell unit is a pure electrical power generator, and not an electrical power load. However, if the current flow direction were to change with electrical current flowing into the fuel cell unit, for example due to faulty operation, then this can lead to damage, or in the extreme even to destruction, of the fuel cell unit. Faulty operation such as this occurs, for example, when a voltage which is greater than the no-load voltage of the fuel cell unit is applied to the output terminals of the fuel cell unit.
The no-load voltage is generally defined as the voltage which is present on the fuel cell unit when the entire fuel cell system is in the no-load mode. The no-load mode is defined such that the fuel cell system is in a state in which it is not emitting any current. If the fuel cell system is used in an electrically powered vehicle, then neither the electrical traction motor for driving the vehicle nor any energy store, which can normally be used as a hybrid energy store and thus as a second power supply unit in addition to the fuel cell system, is supplied with current from the fuel cell unit when in the no-load mode. In consequence, by way of example, in the no-load mode, the electrical traction motor for the vehicle typically does not emit any mechanical drive power to the driven wheels of the vehicle, unless this is alternatively supplied from a battery.
In order to avoid faulty operation and damage to the fuel cell unit, it is therefore necessary to ensure that no current flows into the fuel cell unit from electrical components which are connected to the fuel cell unit and can be operated in a generator mode, that is to say which can generate and/or emit electrical power. This is normally ensured by arranging a diode in the cables, which connects the fuel cell unit to the corresponding electrical components.
One object of the invention is to provide an improved fuel cell system for maintaining the intactness and serviceability of the fuel cell unit of the fuel cell system, and a method for operating the fuel cell system according to the invention.
The fuel cell system according to the invention has a voltage monitoring unit of a redundant configuration and which contains at least two voltage monitoring units. The advantage of a redundant voltage monitoring system is its robustness against faults. If one of the voltage monitoring units is operating incorrectly, then it is possible to make use of the other voltage monitoring unit. A further advantage is that the voltage monitoring units themselves diagnose faulty behaviour in one another by continuously comparing voltage actual values, in particular measured values and/or estimated values.
Furthermore, there is no need for a diode, as is normally provided in the cables which connect the fuel cell unit to the corresponding electrical components, in order to prevent any current from flowing into the fuel cell unit. This leads to a relatively large amount of weight being saved since this type of diode normally has a, precharging apparatus, cooling and a housing, which weighs several kilograms. Furthermore, without the diode there is a reduction in the required space.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.